OneDrive-backed workspaces silently truncate files on write (Cowork)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment

• OS: Windows 11
• Workspace location: OneDrive-synced folder (C:\Users\...\OneDrive\Documents\Obsidian\)
• Cowork version: 2.1.87

Description

Files written or updated by Cowork (via Write/Edit tools) are silently truncated when the file grows in size. No error is thrown. The mounted file ends mid-sentence with no closing newline on subsequent sessions.

Steps to Reproduce

1. Mount a OneDrive-synced folder as a Cowork workspace
2. Have Claude write or update a file that grows in size from its previous version (e.g., appending content to an existing context file)
3. End the session
4. Start a new session and read the same file

Observed Behavior

• File on the Cowork mount ends mid-sentence with no closing newline
• cat -A on the mounted file confirms no trailing $ (no newline) at end
• wc -l vs awk NR line count discrepancy confirms truncation
• No error was thrown during the write operation — the Write tool reported success
• The local Windows copy (viewed in Obsidian) appears complete, suggesting a sync discrepancy between the OneDrive source and the Cowork mount

Expected Behavior

Write operations should either complete fully or fail with a visible error. Silent partial writes that report success are dangerous — subsequent sessions treat the truncated file as the canonical version.

Additional Context

This appears related to OneDrive's Files On-Demand / Cloud Files API behavior with the virtiofs/Plan 9 mount protocol used by Cowork's Linux sandbox. When a file grows during a write, the OneDrive layer may not correctly extend the allocation, resulting in the write being silently clipped to the original file size.

Related issues: #40231 (silent skill file truncation), #29153 (.claude.json corruption cascade on OneDrive), #30928 (EEXIST on OneDrive directories)

What Should Happen?

Write operations should either complete fully or fail with a visible error. Silent partial writes that report success are dangerous — subsequent sessions treat the truncated file as canonical. At minimum, Cowork should detect when a write results in a file smaller than the intended content and surface an error to the user.

Error Messages/Logs

Steps to Reproduce

1. Mount a OneDrive-synced folder (e.g., C:\Users\...\OneDrive\Documents\) as a Cowork workspace
2. Have Claude write or update an existing file such that the new content is larger than the previous version (e.g., appending new sections to a context file like CLAUDE.md)
3. End the Cowork session
4. Start a new session and read the same file using the Read tool or cat -A

Result: The file ends mid-sentence with no trailing newline. The Write tool reported success during the write. wc -c on the mounted file confirms it is smaller than the content that was written. cat -A shows no $ at the end of the last line, confirming no closing newline and an abrupt truncation.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.87 (Cowork)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_